1. Field of the Invention
The present invention relates to modulated-signal communication systems and, more particularly, to signal transition control in such systems.
2. State of the Art
In modulated-signal communications systems, it is well known to impress information on a sinusoidal carrier signal by changing the carrier amplitude, frequency, phase, or combinations thereof. In digital communication systems, the information impressed on the carrier signal is a digital data stream composed of 1s and 0s. One common modulation technique in digital communication systems is phase-shift keying (PSK) in which a phase reversal of the carrier signal of 180.degree. signals a data transition from 1 to 0 or vice versa.
Referring to FIG. 1A, for example, the sequence 1, 0, 1, 0 is to be transmitted using PSK. Each bit has a duration of one and one half cycles of the carrier signal. The initial 1 is therefore represented by one and one half cycles of the carrier signal at a given phase. At the end of the first bit time, a discontinuity occurs as the phase of the carrier signal is reversed to represent the change from a 1 to a 0. Likewise, at the end of the second bit time a discontinuity occurs as the phase of the carrier signal is reversed to represent the change from a 0 to a 1, and so forth.
As is well-known from Fourier analysis, abrupt transitions such as occur between bits in FIG. 1A are inherently wide-bandwidth. In radio communications, radio energy transmitted in connection with a particular signal must be confined within a particular channel bandwidth. Furthermore, the portion of the radio spectrum allocated to any particular use is limited by F.C.C. regulations, within the United States. Therefore, within a limited portion of the radio spectrum, individual channels must be defined so as to have a relatively wide bandwidth if signals of the type shown in FIG. 1A are to be accommodated. As a result, fewer channels can be defined within the limited portion of spectrum than might otherwise be possible and thus less information can be carried.
As described with respect to FIG. 1A, discontinuities can occur in a digitally modulated signal upon transition from one signal state to another. This situation is represented in FIG. 2A, which shows a 2-dimensional signal space. Transitions between the two signal states 0 and 1 occur as "snap transitions", the intervening signal space between the two signal states being traversed almost instantaneously.